Satellite broadcasting systems to mobile receivers have been proposed for radio (xe2x80x9cSatellite DAB,xe2x80x9d International Journal of Communications; Robert D. Briskman; Vol. 13, February 1995, pp. 259-266) and other broadcast services, such as television or data from satellites at 35,786 km altitude located at or near the equatorial plane. These satellites well serve geographical regions at low and mid-latitudes but, as the latitude becomes higher, the elevation angles to the satellites decrease as shown in FIG. 1. High elevation angles are most desirable in satellite broadcast systems using mobile receivers to reduce service outages from physical blockage, multipath fading and foliage attenuation. Recognition of this has led to satellite systems using 12-hour inclined elliptical orbits such as the Molniya communications satellites and the proposed Archimedes radio broadcast system. These systems are not efficient since many satellites are required for continuous coverage of practical service areas and the satellites"" electronics and solar power subsystems are degraded by the four times daily passage through the Van Allen radiation belts surrounding the earth. The systems and methods of this invention surmount these problems.
The systems and methods of this invention use satellites in 24 sidereal hour orbits (geosynchronous) with inclinations, orbital planes, right ascensions and eccentricities chosen to optimize coverage of a particular service area, region or country located at high latitudes. In contrast to the elevation angles of FIG. 1, a satellite constellation of two, three or more satellites can provide during all or most of every day 50xc2x0-60xc2x0 elevation angles throughout a large service area located at high latitudes. The satellites"" orbits can also be configured to avoid most of the radiation from the Van Allen belts.
Satellite systems of this invention, in preferred embodiments, serve geographical latitude service areas located at greater than approximately 30xc2x0 N or 30xc2x0 S by providing high elevation angles to mobile receivers in such areas for reception of broadcasting transmissions over all or most of the day. The preferred systems use geosynchronous satellites (i.e., having a 24 sidereal hour orbital period xe2x80x9486, 164 seconds) in a constellation. The design of the constellation is configured to optimize the elevation angle coverage of a particular geographical high latitude service area for achieving minimum physical blockage, low tree foliage attenuation and small probabilities of multipath fading. For instance, 13 shows an improvement in foliage attenuation at a 1.5 GHz transmission frequency of many decibels for high service reliabilities when the reception elevation angel is doubled. Such dramatic improvement also occur for other Similar improvements occur for other microwave frequencies and for other service reliabilities.
The configuration design optimization is achieved by selection of the orbital parameters of the constellation""s satellites and the number of satellites in the constellation. Satellite audio broadcasting systems to mobile receivers generally provide multichannel radio service and the satellite transmissions are nominally between 1-4 GHz.
Inclination. The inclination of the satellites is generally chosen between about 40xc2x0 and about 80xc2x0 so they cover the desired high latitude service areas during their transit overhead.
Eccentricity. The eccentricity is chosen to have a high apogee over the service area so the satellites spend the maximum amount of time overhead. Practically, the eccentricity is limited by the increased distance that the higher is from the service area since this extra distance must be overcome either by higher satellite transmission power, a more directive satellite antenna during this portion of the orbit or combinations thereof. The eccentricity range in preferred embodiments is from about 0.15 to about 0.30. Eccentricities between about 0.15 and about 0.28 ate highly preferred since they avoid most of the Van Allen belts.
Planes/Number of Satellites. The number of orbital planes equals the number of satellites, and their spacing at the equator is equal to 360xc2x0 divided by the number of satellites. Preferred embodiments have satellite constellations between 2 and 4 satellites. To illustrate, for a 3-satellite constellation, the satellites would be in orbital planes separated by approximately 120xc2x0.
Argument of Perigee. For service to latitude areas above 30xc2x0 N, the argument of perigee is in the vicinity of 270xc2x0 so that the apogee is in the northern hemisphere and the perigee is in the southern hemisphere. For service to latitude areas below 30xc2x0 S, the argument of perigee is in the vicinity of 90xc2x0 so that the apogee is in the southern hemisphere and the perigee is in the northern hemisphere.
Longitude of the Ascending Node. The orbit planes are chosen with a longitude of the ascending node such that the satellites have a good view (i.e., are at high elevation angles as viewed by mobile receivers) of the complete service area. Generally, this is accomplished by choosing the right ascension of the ascending node and the mean anomaly such that the center of the ground trace bisects the service area.
Ground Trace. In the preferred embodiment, the satellites follow the same ground trace and pass over a given point on the earth at approximately equal time intervals. The orbit of each satellite occupies its own orbital plane. For satellites in neighboring planes in a constellation of n satellites, the difference in right ascensions of the ascending nodes is 360xc2x0/n, the difference in mean anomalies is 360xc2x0/n and the average time phasing between the satellites on the trace is 24 sidereal hours/n.
Orbit Control. Satellite constellations of this invention experience change in the aforementioned orbital parameters over time due to the earth""s oblateness, gravity forces of the sun, moon and solar radiation pressure. These can be compensated by the satellites"" on-board propulsion system. The amount of such propulsion can be minimized by analyzing the perturbations of each individual orbit parameter over the lifetimes of the satellites caused by the previously mentioned effects and choosing the initial conditions of the orbits so the minimum on-orbit changes are required. This choice is generally assisted by the fact that some perturbation sources partially cancel out others.
Satellite Spatial and Time Diversity. FIG. 3 shows the elevation angle coverage from Seattle, Wash. to a three-satellite constellation optimized by the methods described herein for broadcast service to the United States of America. Two satellites are visible at all times. The techniques for satellite spatial and time diversity described in U.S. Pat. No. 5,319,672 dated Jun. 7, 1994; U.S. Pat. No. 5,278,863 dated Jan. 11, 1994 and U.S. Pat. No. 5,592,471 dated Jan. 7, 1997 are fully applicable, and these patents are incorporated herein by reference.
The satellite transmission power margin saved by using the invention for mitigation of multipath fading and for reduction of tree and foliage attenuation can be used to advantage. One use is by employing a smaller, less costly satellite. A second use is by transmitting more program channels.